The invention relates to a method and an apparatus for the permanent polarization of piezoelectric bodies, more particularly piezoceramic discs for electroacoustic transducers wherein the bodies are exposed to a polarizing constant electrical field.
The activation of a piezoelectric body in a constant electrical field causes an alignment of the regions of spontaneous polarization present therein. Normally, the spontaneous polarization of all the regions in a piezoelectric substance is arranged randomly so as to be statistically distributed to all directions of space so that no piezoelectric effect acting outwardly becomes apparent. Due to the hysteresis properties of the piezoelectric body, after the disappearance of the activating constant field, there is a resultant residual polarization. All electrical magnitudes, including the electromechanical magnitudes coupled therewith, more particularly the electroacoustic coupling factor, depend on the value of the residual polarization. The latter, in turn, is a function of the field strength of the aforesaid constant electrical field, its period of immersion and the temperature at which polarization occurs.
Normally, piezoelectric bodies are polarized in an oil bath, wherein they are connected with the two electrodes mounted on the two disc surfaces parallel to the plane thereof between two contact pins connected to the polarizing power source.
The oil bath serves as a thermally conducting medium through which the piezoelectric body shall be brought to the desired temperature. Additionally, the oil acts as an insulating material so as to reduce the hazards of voltage breakdowns.
When used in electroacoustic transducers, one of the disc surfaces of the piezoelectric body is connected to a permanently clamped diaphragm or plate. A prerequisite for a reliable and permanent connection is that the disc surface concerned be sufficiently smooth.
It has been demonstrated that disc-shaped piezoelectric bodies frequently have warped surfaces prior to polarization. In general, these warpings have a saddle-shaped configuration. During the polarizing process between two contact pins the warpings deteriorate or other warpings appear, which render the connection with a smooth diaphragm or plate difficult. To remedy the situation, time-consuming sorting operations must be performed and the warped piezoceramic bodies must be made sufficiently smooth to meet the above discussed requirement. If, as usual, the polarization occurs in an oil bath, a cleansing process must also be included, e.g., in an ultrasonic bath. This is costly and time-consuming.
Accordingly, it is an object of this invention of provide a method for the polarization of piezoelectric bodies which renders unnecessary the subsequent smoothing and cleaning processes.